If it was actually them, I'd guess they were banging on the titanium end cap.

I believe the forward end cap (where the porthole is installed) is titanium, and exposed on both sides, so presumably that's what they'd be banging on.

I feel like if there was a way to dampen sounds that well then military submarines would have adopted it long ago. No idea though.

That's what an article emphasized that I skimmed through. The sub had automatic pinging, and systems to automatically raise it to the surface if an untoward event happened. For communication and pinging to be lost, as well as the lack of floating submersible... It's likely whatever happened buggered the whole setup. The banging also was supposedly heard in 30 minute intervals, but no one is saying how many of those intervals happened. The banging could be from anything. The deeps get strange, and the water distorts much of what we know. Multiple people with knowledge from other searches, like the 2014 MH flight, said banging was often heard during their searches but it was always from other sources.